In the prior-art vehicle heating systems of this type, a heat carrier is heated in the heater, and from the heater it reaches a heating heat exchanger, via which the interior space of the vehicle is heated, and it returns from there, usually still with a considerable caloric content, to the vehicle drive engine and from there again to the heater. This prior-art circuit offers the advantage of good preheating of the vehicle drive engine before a cold start in the winter. However, it has the disadvantage that the heat carrier reaches the heater in a greatly cooled state because of the considerable heat losses in the rather long lines and due to the release of heat to the vehicle drive engine, and it must be reheated there. To obtain relatively short times for deicing the windshield in spite of this, it is necessary to use a heater design for an excessively high capacity, or to accept a longer heating. This in turn means power consumption from the automobile battery for a longer time and a higher fuel consumption.
A vehicle heating system, in which the heating of the interior space of the vehicle and the deicing of the windshield are given priority, has been known from DE 39 20 505 A1. This is achieved by the heat carrier circuit having a connection line, which leads from the return to the first part of the flow pipe, which first part is located between the vehicle drive engine and the heater, so that a short circuit is formed, which passes through the heater and the heat exchanger, bypassing the vehicle drive engine, and by providing a current-actuated valve for the first part of the flow pipe or for the second part of the return, which is located between the branch of the connection line and the vehicle drive engine, wherein the current-actuated valve is controlled by the control device of the heater.
It is achieved in this prior-art vehicle heating system that the entire caloric content of the heat carrier arriving from the heater is available for the heating heat exchanger, aside from fairly minor heat losses to the environment, when the current-actuated vane is closed. The current-actuated vane opens completely or partially, thereby releasing the flow path of the heat carrier through the internal combustion engine, only when the heating heat exchanger is no longer able to supply completely enough the total amount of available heat to the interior space of the vehicle, i.e., when the heat carrier temperature increases as a consequence excessively on the outlet side.
However, this vehicle heating system is very expensive and can be retrofitted only at a great expense, and it also has the major disadvantage that power from the automobile battery is necessary for controlling the circuit, precisely at low temperatures, at which the automobile battery has a low capacity. An additional battery has therefore also been used to nevertheless supply a sufficient amount of electricity for the starting process of the vehicle.